Coupling between carrier and transmission lines



July 28, 1931.

E. l. GREEN COUPLING BETWEEN CARRIER AND TRANSMISSION LINES Filed Nov.2, 1926 INVENTOR. I. Greew ATTORNEY Patented July 28, 1931 ESTILL I.GREEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OF NEW YORK COUPLING BETWEEN CARRIERAND TRANSMISSION LINES Application filed November 2, 1926. Serial No.145,899.

This invention relates to transmission lines and more particularly toarrangements for improving the efliciency of coupling between carrierapparatus and transmission 5 lines.

The arrangements of the invention are particularly applicable to caseswhere it is impossible or undesirable to associate carrier apparatusdirectly with the line circuit over which the carrier transmission isaccomplished, and where some form of coupling device such as a loop orantenna must therefore be employed. Such cases frequently arise inconnection with power line carrier systems, in many of which a couplingantenna or loop is employed in order to separate the carrier apparatusfrom the high voltage on the power lines. A further field of applicationfor the arrangements of the invention may be in connection with carriersystems for communication with moving vehicles, such as trains, where itis impossible to connect the carrier apparatus directly to thetransmission line.

It has been found that coupling devices, such as a loop or antenna,utilized in arrangements of the above type will ordinarily haveconsiderable capacity to ground, and, when transmitting to a groundreturn circuit or to a metallic wire circuit which has appreciablecapacity to ground, this capacity of the coupling device will constitutea source of loss. It is the primary object of the arrangements of thisinvention to utilize loading to raise the impedance of'the couplingdevice to ground, thereby annulling the undesired effect of thiscapacity to ground and greatly increasing the efficiency of the couplingdevice. Other ob ects, features, and applications of the lnvent on willappear more fully from the detailed description hereinafter given.

The invention may be more fully understood from the followingdescription together with the accompanying drawings in the Figures 1, 2,3, 4: and 5 of which the invention is illustrated. In Fig. 1 is shown apower line having carrier apparatus cow led thereto. Fig. 2 is aschematic equivaent of Fig. 1. In Fig. 3 is shown carrier apparatuscoupled to a power line with the arrangements of this invention. Figs. 4and 5 illustrate modifications of the invention. Similar referencecharacters have been utilized to denote like parts in all of thefigures.

A common method of coupling carrier apparatus to a power line is shownschematically in Fig. 1. The carrier apparatus is connected through asuitable transformer T to a coupling wire or antenna CC, whereby thecarrier current may be transmitted to the power line LL by electrostaticinduction. The power line LL, which will ordinarily consist of severalwires, is shown as terminating at the sending end in the impedance toground, shown as Z, of the transformer which supplies power to the wiresLL. At the distant end the power line may be assumed for a long line atcarrier frequencies to be terminated in the characteristic impedance toground Z For practical purposes, this characteristic impedance Z becomesat carrier frequencies a pure resistance.

In Fig. 2 is shown the approximate equivalent of the arrangements shownin Fig. 1. The impedance looking back into the transformer T is shown asconsisting of the inductance L and resistance R. The capacity of thecoupling wire to the power wires is shown as C the capacity of thecoupling wire to ground is shown as C and the power transformerimpedance to ground Z is shown as a capacity G with a loss, ordinarilysmall, represented by 7.

For the, circuit of Fig. 2 the efliciency is the ratio of the powerutilized in Z to the total power delivered to the circuit. Neglectingfor the moment the power transformer capacity and loss, it can be shownthat to obtain maximum efiiciency the following three conditions shouldbe fulfilled.

First, R Z (1) where R is the resistance component looking into thetransformer T, and Z is the characteristic impedance of the power wireto ground. In other words, the resistance components of the impedance ofthe generating device and of the load impedance should be matched.

Second,

where w is equal toQmtimesthe frequency at which maximum efliciency isdesired. This would amount to annulling the reactances, or in otherwords, properly tuning the circuit arrangement for the workin carrier'frequency. It may be pointed out that the principles embodied in theabove two conditions are well known with respect to circuits in general.Thus it is generally stated that the maximum power is delivered from theterminals of a generating device which has a given generated voltage anda fixed internal resistance and reactance when the external resistanceequals the internal resistance, and the external reactance is equal tothe internal reactance but of opposite sign. However, the specialapplication of these prin- 'ciples in order to materially increase theelliciency of the coupling in a circuit similar to that of Fig. 2 hasnot been previously pointed out.

The third condition for maximum efliciency is that an additionalinductance L (shown in dottedlines) should be connected in parallel withC and that L should be chosen so that which involves the use of meansnot shown in Fig. 1 to obtain an anti-resonant circuit be-' tween thecoupling wire and ground, is one of the primary objects of thisinvention. The power transformer capacity which was neglected may havesome slight effect on the validity of the first two conditions, but canhave none of the third. From a physical standpoint the third conditionamounts to giving the coupling wire a very high impedance to ground sothat most of the energy will choose the low impedance path to the powerline. This high impedance to ground is obtained by loading the couplingwire with one or more inductance elements, such as L and L as shown inFig.3. The spacing of the sirable to utilize the loading arrangementsshown in Fig. 4. In this case two loading networks, such as K and Kwould be congive two difi'erent points of anti resonance with the wirecapacity.

Another method of coupling, in which the coupling wire CC is grounded atthe dis, tant end, is shown in Fig. 5.; This is obviously a type ofelectromagnetic induction, in contrast to the electrostatic methodheretofore illustrated. Here the effect of the capacity of the couplingwire to ground will be to shunt off the current, and the efiiciency ofthe arrangement will be'improved by raising the impedance to, ground sothat all the current enters the ground at O. This is accomplished asheretofore explainedby the loading devices L and L Coupling deviceswhich utilize a combination of electrostatic and electromagneticcoupling may be similarly loaded.

The same general plan of loading may be from those described. Forexample, when a metallic loop is used instead of a ground wire loop thecapacity of the loop will still introduce loss which can be eliminatedby loading. Likewise, when the coupling is made with a metallic circuiton the transmission line this circuit will have considerable capacity toground, so that it is still advantageous to eliminate the effect of thecapacity of the coupling device to ground. In every case the loading isentirely independent of other expedients, such as tuning the couplingdevice, matching impedance, etc.,' which are also employed to improvethe efliciency of the system for a particular fre-' quency orfrequencies. Accordingly, while the invention has been described asembodied in certain specific arrangements which are deemed desirable, itis undertood that it is capable of embodiment in many and other widelyvaried forms without departing from the spirit of the invention asdefine in the appended claims. What is claimed is:

1. A transmission line, carrier apparatus,

a coupling device associated with said carthe impedance looking from thecoupling device into the transmission line.

2. A transmission line, carrier apparatus, a coupling device associatedwith said carrier apparatus'and said line whereby carrier currents maybe superimposed on said line from said carrier apparatus, means forraising the impedance to ground of said coupling device at substantiallythe carrier frequencies utilized, and means in said coupling device forso adjusting the impedanceof the carrier source with respect to theimpedance looking from the coupling device into the transmission linethat the resistance components of the two impedances are substantiallyequal and the reactance components of the two impedancesare'substahtially equal in magnitude but of opposite sign. I

In testimony whereof, I have signed my name to this specification this1st day of November 1926.

ESTILL I. GREEN.

